1. Field of the Invention
The present invention relates to a method for compensating picture quality of an image signal, and more particularly to an apparatus and method for eliminating noise of a received image and improving its sharpness.
2. Description of the Related Art
Generally, a moving picture receiving system such as a TV includes a transmitter and a receiver. The transmitter converts a light energy sensed through a camera to an electrical signal and transmits the signal through analog or digital signal processing and channel coding. The receiver processes inverse steps of the transmitter to display a moving picture in a display device. The image displayed through a series of steps has noise due to various reasons and sharpness of the image is lowered due to limited bandwidth of transmission channels and physical characteristics of devices. Those noise and low sharpness cause a deterioration of picture quality. In order to solve the problem as described above, various methods for eliminating noise and improving sharpness have been developed. In general, it is known that noise in the natural world has white Gaussian characteristic. The noise in a frequency domain is uniformly distributed in a whole frequency band. On the other hand, the image signal is characterized in that most of power is concentrated in a low frequency area and power is rapidly reduced in a high frequency area. Accordingly, noise can be eliminated by low pass filtering as shown in FIG. 1.
However, as shown in FIG. 1, it is noted that high frequency components of the image signal are eliminated when the noise is eliminated. This deteriorates sharpness of the image.
Therefore, methods for eliminating noise while preserving high frequency components of the image have been suggested. Of them, there is a double smoothing method which is a low pass filtering method through two steps as shown in FIG. 2. That is, an input signal X(t) is input to a first delay 201 and a first low pass filter (LPF) 202 so that it is delayed by the first delay 201 and at the same time primarily low pass filtered by the first LPF 202. The primarily low pass filtered signal L(t) is output to a subtractor 203 and a second delay 205.
The subtractor 203 outputs a difference signal E(t) between a signal delayed by the first delay 201 and a signal primarily low pass filtered by the first LPF 202 to a second LPF 204. The second LPF 204 low pass filters the difference signal E(t) and outputs the low pass filtered signal to an adder 206. The adder 206 adds the low pass filtered signal of the second LPF 204 to a signal delayed by the second delay 205.
In other words, the double smoothing method of FIG. 2 is performed in such a manner that an image signal is primarily low pass filtered and a difference signal E(t) between an original image signal and the primarily low pass filtered signal is secondarily low pass filtered. A final output signal Y(t) is obtained by adding the secondarily low pass filtered signal N(t) to the primarily low pass filtered signal L(t). The first and second delays 201 and 205 respectively delay data as much as latency of the first LPF 202 and the second LPF 204.
FIGS. 3a to 3e show a method for eliminating noise when a unit step signal having noise is processed by the double smoothing method.
As shown in FIGS. 3a to 3e, the double smoothing method has a problem in that sharpness is deteriorated because the edge of the image is smoothed when eliminating noise.
Meanwhile, sharpness of the image in view of visual characteristic of human being is greatly varied depending on contrast of an outline portion. That is to say, sharpness is high as contrast of the edge region is great. This is obtained by amplifying high frequency components of the image. For example, if the high frequency components are amplified in a signal having abrupt brightness change, over shoot and under shoot occur around the outline, thereby increasing contrast.
There are methods for making the outline sharp, such as a second derivative method, a high pass filtering method, and an unsharp masking method.
Of them, the unsharp masking method has a structure of FIG. 4. That is, an original signal X(t) is delayed by a first delay 401 and at the same time low pass filtered by an LPF 402. A subtractor 403 extracts a difference signal E(t) between the delayed original signal and the low pass filtered signal L(t) and outputs the difference signal E(t) to a multiplier 404. At this time, the difference signal corresponds to high frequency components of the image. The multiplier 404 multiplies the high frequency components by xcex1 and then amplifies the resultant value. The amplified value is added to the original signal delayed by the first delay 401 by an adder 405. Thus, an image having fine sharpness can be obtained.
FIGS. 5a to 5e show a method for contrasting the outline by processing a unit step signal by the unsharp masking method to improve sharpness of the image.
The unsharp masking method has a problem that while improving sharpness, it also amplifies noise.
As described above, it can be recognized that the method for eliminating noise and the method for improving sharpness require contrary functions. That is, if noise is eliminated, the outline of the image is smoothed too, thereby deteriorating sharpness. If filtering is performed to improve sharpness, sharpness is obtained but noise is also amplified. The related art methods suggest algorithms targeting only one of the two missions, i.e., sharpness improvement or noise reduction. Accordingly, the two algorithms for sharpness improvement and noise reduction are sequentially applied, there is a problem that noise reduction performance and sharpness improvement performance are attenuated. Particularly, in case of deteriorating picture quality for the edge region, visual quality is deteriorated.
Accordingly, the present invention is directed to an apparatus and method for compensating an image signal that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an apparatus and method for compensating an image signal in which noise of an image signal is eliminated and an outline becomes sharp using one parameter by unifying algorithms for eliminating noise and making an outline sharp.
Another object of the present invention is to provide an apparatus for compensating an image signal in which the cost for implementing hardware is reduced by sharing a common portion in algorithms for eliminating noise and making an outline sharp.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an apparatus for compensating an image signal according to the present invention is characterized in that noise is eliminated and an outline becomes sharp by sharing a common portion in a structure for eliminating noise by a double smoothing method and a structure for making an outline sharp in an unsharp masking method.
The apparatus for compensating an image signal according to the present invention includes a first filter for low pass filtering an input signal, a subtractor for obtaining a difference signal between the input signal and the low pass filtered signal, a noise eliminating unit for eliminating noise while preserving high frequency components of an image from the difference signal, a multiplier for multiplying an output of the noise eliminating unit by a constant (xcex1+1), and an adder for adding the low pass filtered signal to the output of the multiplier.
In another aspect, a method for compensating an image signal according to the present invention is characterized in that noise reduction and edge sharpening are controlled by one parameter.
The method for compensating an image signal according to the present invention comprises the steps of low pass filtering an input signal, obtaining a difference signal between the input signal and the low pass filtered signal, eliminating noise only while preserving high frequency components of an image from the difference signal, multiplying the difference signal having no nose by a constant (xcex1+1), and adding the low pass filtered signal to the multiplied signal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.